1. Field of the Invention
The present invention relates to a lithium ion storage device.
2. Description of the Related Art
Lithium ion storage devices such as lithium ion capacitors and lithium ion batteries advantageously have a high energy density, a low self-discharge rate, and good cycle performance. Therefore, in recent years, it has been expected to increase the size and the capacity of non-aqueous electrolyte storage devices to use such storage devices as power sources for automobiles such as hybrid vehicles and electric vehicles. In some lithium ion storage devices, a negative plate is doped with lithium ions in advance (lithium pre-doping) to expand the voltage range of use and to increase the capacity.
One method for doping a negative plate with lithium ions in advance is, for example, to place a lithium metal plate in an electrode group before doping, dissolve the lithium metal plate to generate lithium ions, and dope a negative active material of the negative plate with the generated lithium ions. However, if the folded lithium metal plate is wound in an electrode group, a part of the lithium metal plate may pierce a separator to contact a positive plate, which may cause a short circuit between the lithium metal plate and the positive plate.
In a conventional lithium ion storage device as disclosed in Japanese Patent Application Publication No. 2010-212266 (JP2010-212266A) a lithium metal plate is held on a metal foil which is a conductive shielding member to prepare a layered member. A positive plate is divided into two pieces, which are arranged with a predetermined gap therebetween. The layered member holding the lithium metal plate is disposed at a position corresponding to the gap between the two positive plate pieces. That is, the layered member is disposed at a position not facing the two positive plate pieces.
In the conventional structure, the two positive plate pieces are disposed with a predetermined gap therebetween, that is the lithium metal plate is disposed between the positive plate pieces or at a position not facing the positive plate pieces, a portion of the lithium metal plate will not pierce or pass through the separator and will not contact the positive plate pieces, thereby preventing short circuit between the lithium metal plate and the positive plate. However, it is difficult to dispose the lithium metal plate at a position not facing the two positive plate pieces. In addition, burrs produced at cut end surfaces of the two divided positive plate pieces may pierce the separator to contact the negative plate, which may cause a short circuit.